Image display apparatus

ABSTRACT

An image display apparatus has: a display element for displaying an image; a light source section for supplying an illumination light to the display element; a reflection type hologram for diffracting and reflecting the illumination light so as to guide the illumination light to the display element; and an eyepiece optical system for guiding an image light from the display element to an eye of a viewer so as to provide an enlarged virtual image of the image. The reflection type hologram has diffusing properties. Since the reflection type hologram diffuses the illumination light, an incident angle of the illumination light with respect to the display element is widened to a certain extent. Since the display element does not change a propagating direction of a light ray, a propagating direction of the image light is widened similarly to widening of the incident angle, thereby enlarging the observation pupil.

This application is based on the application No. 2002-197518 filed inJapan, Jul. 5, 2002, the entire content of which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image display apparatus having adisplay element in which illumination light is modulated into lightshowing an image. Particularly, the invention relates to an illuminationoptical system of the image display apparatus.

2. Description of the Related Art

In recent years, a head mounting type image display apparatus which ismounted to a head and is used in front of eyes spreads as an imagedisplay apparatus for personal use. In general, the head mounting typeimage display apparatus has a display element for displaying an image,and an eyepiece optical system for guiding light of the image displayedon the display element to eyes of a viewer and providing an enlargedvirtual image. Although CRT (Cathode-Ray Tube), for example, in whichthe element itself emits light showing an image is occasionally used asthe display element, a liquid crystal display, in which the displayelement for modulating given illumination light into light showing animage (hereinafter, referred to as an image light) is combined with asmall light source (for example, a light emitting diode) for supplyingthe illumination light, is mostly used in order to miniaturize andlighten the apparatus.

A structure, such that the light source is combined with the displayelement for modulating the illumination light from the light source, isprovided with a diffracting reflection element for diffracting andreflecting the illumination light from the light source so as to guideit to the display element in order that while a degree of freedom ofarrangement positions of the light source, the display element and theeyepiece optical system is being heightened, an incident angle of theillumination light with respect to the display element is made to besuitable.

The structure of such an image display apparatus is schematically shownin FIG. 17. The image display apparatus is composed of a liquid crystaldisplay 91 which is the display element, a light emitting diode 92 whichis the light source, a diffracting reflection element 93, an eyepieceoptical system 94, and a convex lens 95. Divergence light emitted fromthe light emitting diode 92 as illumination light is changed intoapproximately parallel light by the convex lens 95. The diffractingreflection element 93 diffracts and reflects the illumination lightwhich is changed into the approximately parallel light by the convexlens 95 at a reflection angle different from the incident angle so as toguide the light to the liquid crystal display 91. The liquid crystaldisplay 91 guides the illumination light in the form of theapproximately parallel light as an image light to the eyepiece opticalsystem 94, and the eyepiece optical system 94 provides an enlargedvirtual image of the image to an eye E of the viewer. As the diffractingreflection element 93, a diffraction grating or a hologram is used.

A prior image display apparatus for guiding the illumination light tothe eyepiece optical system through the diffracting reflection element,however, has a small observation pupil because the illumination lightguided to the display element is parallel light or is extremely close tothe parallel light. The eye of the viewer is, therefore, easily deviatedfrom the observation pupil of the apparatus due to a change in arelative position between the apparatus and the eye of the viewer, and apart of an image to be observed is omitted or the image cannot beobserved completely. The head mounting type image display apparatusdesirably provides easy mounting and has agreeability, and it is mostlyof a glass type. In this form, however, the apparatus easily moves,thereby a situation where the image cannot be observed arisesfrequently.

Even if a member for fitting is devised so as not to move the apparatus,since a gap between left and right eyes differs depending on individualsand the relative position between the apparatus and the eye differsdepending on viewers, it is difficult to provide the image displayapparatus with high general-purpose properties. In order to provide theapparatus with which many people easily observe an image, it isnecessary that the entire optical system including the light sourcethrough the eyepiece optical system is movable with respect to anmounting member, and thus the structure becomes complicated, and theminiaturization and lightening become difficult.

SUMMARY OF THE INVENTION

In order to solve the above problems, it is an object of the presentinvention to provide an image display apparatus having a largeobservation pupil. Another object of the present invention is to providean image display apparatus having a large observation pupil in whichillumination light can be utilized efficiently for observation of animage.

In order to achieve the above objects, according to the presentinvention, in an image display apparatus comprising: a light sourcesection for supplying an illumination light; a display element formodulating a given illumination light into an image light showing animage; a reflection type hologram for diffracting and reflecting theillumination light from the light source section so as to guide theillumination light to the display element; and an eyepiece opticalsystem for guiding the image light from the display element to an eye ofa viewer so as to provide an enlarged virtual image of the image. Thereflection type hologram has diffusing properties, and while diffractingand reflecting the illumination light, diffuses the illumination lightso as to guide the diffused light to the display element.

Since the reflection type hologram not only diffracts and reflects theillumination light so as to change its propagating direction but alsodiffuses the illumination light, an incident angle of the illuminationlight with respect to the display element is widened to a certainextent. Since the display element modulates the illumination light butdoes not change a propagating direction of a light (if a transmissiondisplay element, the incident angle is equal with an emergence angle,and if a reflection type display element, the incident angle is equalwith a reflecting angle), a propagating direction of the image light iswidened similarly to widening of the incident angle, thereby enlargingthe observation pupil.

In order to achieve the above objects, according to another invention,an image display apparatus comprising: a light source section forsupplying the illumination light; a display element for modulating agiven illumination light into an image light showing an image; a firstreflection type hologram for diffracting and reflecting the illuminationlight from the light source section so as to guide the illuminationlight to the display element, the first reflection type hologram havingdiffusing properties; a second reflection type hologram for guiding animage light from the display element to an eye of a viewer so as toprovide an enlarged virtual image of the image; and a transparentplate-shaped prism for holding the second reflection type hologram. Theimage display apparatus is used with the second reflection type hologramplaced in front of the eye of viewer and with the transparentplate-shaped prism facing the eyes. The image light from the displayelement enters the transparent plate-shaped prism at an end surfacethereof so as to be guided to the second reflection type hologram and isguided to the eye of the viewer by the second reflection type hologramas an enlarged virtual image of the image, and a light from an outerworld transmits through the plate-shaped prism and the second reflectiontype hologram so as to be guided to the eye, thereby the viewer cansimultaneously see an image of the outer world and the virtual image.And the viewer can see the virtual image with a large observation pupilbecause of diffusing properties of the first reflection type hologram.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the preferred embodiments with the reference to theaccompanying drawings in which:

FIG. 1 is a diagram schematically showing an optical structure of animage display apparatus according to a first embodiment;

FIG. 2 is a diagram schematically showing a method of manufacturing areflection type hologram provided to the image display apparatusaccording to the first embodiment;

FIG. 3A shows an example of a relationship between a wavelength of laserbeam for exposure and absorptance of photosensitive materials in thecase where two kinds of hologram photosensitive materials are used;

FIG. 3B is a diagram schematically showing a relationship between awavelength and diffracting reflection efficiency in the obtainedreflection type hologram;

FIG. 4 is a diagram schematically showing the optical structure of theimage display apparatus according to a second embodiment;

FIG. 5 is a diagram schematically showing the method of manufacturingthe reflection type hologram provided to the image display apparatusaccording to the second embodiment;

FIG. 6 is a diagram schematically showing the optical structure of theimage display apparatus according to a third embodiment;

FIG. 7 is a diagram schematically showing the optical structure of theimage display apparatus according to a fourth embodiment;

FIG. 8A is a diagram showing a sectional shape of illumination light inthe case where the reflection type hologram provided to the imagedisplay device according to the embodiments has isotropic diffusingproperties;

FIG. 8B is a diagram showing an example of the sectional shape of theillumination light in the case where the reflection type hologram hasanisotropic diffusing properties;

FIGS. 9A and 9B are diagrams showing a size of an observation pupil whenthe reflection type hologram has the diffusing properties of FIG. 8B anda position relationship between the observation pupils and eyes of aviewer;

FIGS. 10A through 10D are diagrams schematically showing another methodsof manufacturing the reflection type hologram provided to the imagedisplay apparatus according to the embodiments;

FIG. 11 is a diagram schematically showing a part of the opticalstructure of the image display apparatus according to a fifthembodiment;

FIG. 12 is a diagram schematically showing a method of manufacturing thereflection type hologram provided to the image display apparatusaccording to a fifth embodiment;

FIG. 13 is a diagram schematically showing the entire optical structureof the image display apparatus according to the fifth embodiment;

FIG. 14 is a diagram schematically showing a method of manufacturing thereflection type hologram as an eyepiece optical system provided to theimage display apparatus according to the fifth embodiment;

FIG. 15 is a perspective view showing an outline when the image displayapparatus according to the fifth embodiment is of a glass type;

FIG. 16 is a sectional view of the image display apparatus according tothe fifth embodiment; and

FIG. 17 is a diagram schematically showing the optical structure of aprior image display apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of an image display apparatus of the present invention willbe explained below with reference to the drawings. An optical structureof the image display apparatus 1 according to a first embodiment isschematically shown in FIG. 1. The image display apparatus 1 has aliquid crystal display 11, a light emitting diode (LED) 12, a reflectiontype hologram 13, an eyepiece optical system 14 and a convex lens 15.

The liquid crystal display 11 has a plurality of pixels and is applied avoltage to the pixels so as to be capable of changing a state of aliquid crystal layer. When an electric signal based on image data isinput into the liquid crystal display 11, the liquid crystal display 11modulates given illumination light into an image light. The LED 12supplies the illumination light to be given to the liquid crystaldisplay 11. The reflection type hologram 13 diffracts and reflects theillumination light given from the LED 12 via the convex lens 15 andguides 1st-order diffracted reflection light to the liquid crystaldisplay 11. The eyepiece optical system 14 guides the image light fromthe liquid crystal display 11 to an eye E of a viewer, and provides anenlarged virtual image of the image on the liquid crystal display 11.The convex lens 15 changes divergent light L1 from the LED 12 intoapproximately parallel light L2 so as to give it to the reflection typehologram 13.

The reflection type hologram 13 has diffusing properties, and diffractsand reflects the illumination light L2 from the convex lens 15 andsimultaneously diffuses it. An incident angle of illumination light L3with respect to the liquid crystal display 11, therefore, has adistribution. The liquid crystal display 11 does not influence atraveling direction of the light, and the image light comes out of thepixels of the liquid crystal display 11 at an outgoing angle which isthe same as the incident angle so as to reach the eyepiece opticalsystem 14. The observation pupil of the image display apparatus 1 is,therefore, large, and even if a relative position between the imagedisplay apparatus 1 and the eye E of the viewer fluctuates, the eye E ofthe viewer is difficultly deviated from the observation pupil.

In order to provide a bright image, the convex lens 15 and the eyepieceoptical system 14 are set so that the LED 12 and the eye E of the viewerare optically conjugated. The LED 12 is small and is close to a pointlight source. Since the size of the observation pupil is approximatelyproportional to an area of the light source, when the reflection typehologram 13 simply diffracts and reflects the illumination light L2 fromthe convex lens 15, the observation pupil is small. When the diffusingproperties are, however, provided to the reflection type hologram 13,the apparatus is in a state such that a large surface light source 12 iis provided in order to illuminate the liquid crystal display 11,thereby realizing the large observation pupil.

A method of manufacturing the reflection type hologram 13 having thediffusing properties is schematically shown in FIG. 2. A photosensitivematerial 13 b is applied or is bonded to a transparent substrate 13 cmade of glass, acrylic or the like, so that a primary body 13 a of thehologram 13 is obtained. A transmission diffusing plate 16 is arrangedon the primary body 13 a on the side of the substrate 13 c, and twolaser beams La, Lb, which are obtained by branching a laser beam emittedfrom the same laser beam source, are emitted from the side of thephotosensitive material 13 b and the side of the diffusing plate 16,respectively, and interference bands of the laser beams La, Lb arerecorded on the photosensitive material 13 b. The laser beam La isapproximately parallel light similarly to the illumination light L2 fromthe convex lens 15, and is emitted from the same direction as theillumination light L2. The laser beam Lb is also approximately parallellight, and is emitted from a direction where the diffracted andreflected illumination light L3 is extended to an upper stream side.

The laser beam Lb transmits through the diffusing plate 16 so as to bediffused, the interference bands of the laser beam La and the diffusinglaser beam Lb are recorded on the photosensitive material 13 b, so thatthe reflection type hologram 13 having diffusing properties similar tothe diffusing plate 16 can be obtained. A type of the photosensitivematerial 13 b is not particularly limited, and general materials such assilver salt, dichromic acid gelatin and photopolymer can be used.Particularly the photopolymer can be subject to a post-treatment afterthe recording of the interference bands according to a dry process, andthus it is preferable from a viewpoint of treatment efficiency.

As the transmission diffusing plate 16, a plate in which a surface of atransparent substrate is roughed, or a plate in which a diffusing resinis mixed with an optically transparent material which is conventionallyknown can be used. In the latter one, however, since reflected light isincreased and transmitted light is reduced topically, the former one ispreferable.

It is also considered that the transmission diffusing plate is arrangedon an optical path including the LED 12 through the liquid crystaldisplay 11, and the illumination light is diffused not by the reflectiontype hologram 13 but by the transmission diffusing plate so that theobservation pupil is enlarged. On the diffusing plate in which thesurface of transparent substrate is roughed, however, uneven intensityof the transmitted light easily occurs due to the convexo-concave shape,and when the diffusing plate is arranged in a vicinity of the liquidcrystal display, uneven brightness occurs on the light showing theimage, thereby deteriorating quality of the image to be provided. On thetransmission diffusing plate in which the diffusing resin is mixed,light is mostly lost by reflection, thereby greatly deteriorating useefficiency of the light from the LED 12.

When the hologram is provided with the diffusing function, the plate inwhich the surface of the transparent substrate is roughed is used as thetransmission diffusing plate 16 at the time of the manufacturing, and asshown in FIG. 2, the transmission diffusing plate 16 and thephotosensitive material 13 b are not bonded but separated, therebyrecording a state such that the uneven intensity due to the unevensurface of the transmission diffusing plate 16 is mitigated on thephotosensitive material 13 b. That is to say, the reflection typehologram 13 is characterized in that it is equivalent to thetransmission diffusing plate 16 from a viewpoint of a diffusing angle,but it has higher uniformity than the transmission diffusing plate 16from a viewpoint of the intensity of the diffused light to be provided.In the image display apparatus 1 in which the illumination light isdiffused by the reflection type hologram 13, therefore, the image withhigher quality and less uneven brightness can be provided in comparisonwith the case where the illumination light is diffused by the priortransmission diffusing plate in which the surface of the transparentsubstrate is roughed.

In order to provide a color image, red light (hereinafter, referred toas R light), green light (hereinafter, referred to as G light) and bluelight (hereinafter, referred to as B light) may be guided as theillumination light to the liquid crystal display 11. In this case, threephotosensitive materials which selectively have sensitivity with respectto the R light, G light and B light are laminated on the substrate, andlaser beams of the R light, the G light and the B light are emittedindividually, so that the interference bands are formed on thephotosensitive materials. As a result, the high diffracting reflectionefficiency can be obtained. In another method, a single photosensitivematerial having the sensitivity with respective to the R light throughthe B light is used, so that the laser beams of three colors may beemitted. As a result, a cost can be suppressed. Two photosensitivematerials can be used, or holograms which are manufactured individuallycan be laminated. At the time of use, three kinds of the LEDs 12 foremitting the R light, the G light and the B light are used.

As the three-color laser beams, the laser beams having wavelengthscloser to wavelengths of the three-color illumination light at the timeof use are used. FIG. 3A shows a relationship between the wavelength ofthe laser beam and absorptance of the respective photosensitivematerials when the photosensitive material having the sensitivity to theR light, and the photosensitive material having the sensitivity to the Glight and the B light are used so as to manufacture the reflection typehologram 13. FIG. 3B shows a relationship between the wavelength in theobtained reflection type hologram 13 and the diffracting reflectionefficiency. As a result, the reflection type hologram 13 has peaks ofthe diffracting reflection efficiency with respect to the R light, Glight and the B light emitted from the three kinds of the LEDs 12, sothat the image display apparatus 1 which is capable of providing abright image with excellent color balance can be obtained.

The three LEDs 12 for supplying the three-color illumination light maybe arranged in a sheet of FIG. 1, or may be arranged in a verticaldirection with respect to the sheet. In both the cases, the laser beamLa on the side of the photosensitive material 13 b is emitted from thesame direction as the illumination light from the LEDs 12 in order toavoid lowering of the diffracting reflection efficiency at the time ofuse. When this condition is considered, the three LEDs 12 is desirablyarranged in a plane vertical to a line for connecting a center of theliquid crystal display 11 and a center of the reflection type hologram13 so as to be equally separated from the line. This because thedirections of the laser beams La, Lb are made to be uniform, and theprimary body 13 a is only turned so that the above condition issatisfied, thereby facilitating the manufacturing and simplifying astructure of a hologram exposing device.

In the image display apparatus 1 of the embodiment, the transmissiontype liquid crystal display is used as the display element, but thereflection type liquid crystal display can be used. Another type of thedisplay element having a different modulating principle can be used. Asthe light source, besides the LED, an electroluminescence element(hereinafter, referred to as an EL element) can be used. The reflectiontype hologram 13 having the diffusing properties is provided so that thelarge observation pupil can be realized even if the light source issmall. Importance is, therefore, attached to the properties of theelement such as light emitting intensity and a life, and the lightsource may be selected.

Although the eyepiece optical system 14 is composed of a single convexlens, it may be composed of a plurality of lenses, a reflection surface,a prism having axially asymmetric curved surfaces, a reflection typehologram, a transmission type hologram, a diffraction optical element(hereinafter, referred to as DOE) or the like. In order to realizeminiaturization and lightening so as to cope with the large observationpupil, however, it is desirable that the eyepiece optical system 14includes the hologram or the DOE. These points are applied similarly tofollowing embodiments.

FIG. 4 schematically shows the optical structure of the image displayapparatus 2 according to a second embodiment. In the image displayapparatus 2 modified by the image display apparatus 1, a positiveoptical power is provided to the reflection type hologram 230 so thatthe convex lens 15 is omitted. The reflection type hologram 230 stillhas the diffusing properties, and the image display apparatus 2 has thelarge observation pupil similarly to the case where the liquid crystaldisplay 11 is illuminated by a large surface light source 12 i.

The optical power of the reflection type hologram 230 is set so that, ifit does not have the diffusing properties, the divergent light L1 fromthe LED 12 is changed into the approximately parallel light, in otherwords, the diffracted and reflected illumination light L3 whosediffusing angle is 0° is changed into the approximately parallel light.As a result, the illumination light L3, which is guided from thereflection type hologram 230 to the liquid crystal display 11, becomesequivalent to that in the image display apparatus 1.

The method of manufacturing the reflection type hologram 230 isschematically shown in FIG. 5. A difference from the manufacturingmethod of the first embodiment (FIG. 2) is only a point that the laserbeam La emitted from the side of the photosensitive material 230 b ischanged into the divergent light. In order to change the laser beam Lainto the divergent light, the laser beam from the laser beam source is aparallel light with small diameter and it is allowed to pass through apinhole, or is transmitted through the convex lens or a concave lens.The pinhole and the lenses are arranged on the position where the LED 12is arranged at the time of use.

In the embodiment, the light source section supplies the divergent lightas the illumination light, and the reflection type hologram has thepositive optical power, and the diffracted and reflected illuminationlight whose diffusing angle is 0° is the approximately parallel light.When the illumination light whose diffusing angle is 0° is theapproximately parallel light, while the illumination light is beingdiffused, most part of the illumination light can be guided to thedisplay element, so that the illumination light from the light sourcecan be utilized efficiently for providing an image. A small element suchas the light emitting diode can be used as the element for emitting theillumination light, and it is not necessary to additionally provide anoptical element for changing the illumination light as the divergentlight into the approximately parallel light, thereby facilitating theminiaturization and the lightening.

The optical structure of the image display apparatus 3 according to athird embodiment is schematically shown in FIG. 6. In the image displayapparatus 3 modified by the image display apparatus 2 according to thesecond embodiment, 1st-order diffracted reflection light and 0th-orderdiffracted reflection light of the reflection type hologram 330 havingpositive optical power are separated securely from each other, and theliquid crystal display 11 is arranged in a vicinity of a position wherethe 1st-order diffracted reflection light and the 0th-order diffractedreflection light are separated.

The illumination light L1 from the LED 12 is the divergent light, andthe 0th-order diffracted reflection light does not receive the opticalpower of the reflection type hologram 330 so as to be still thedivergent light. The 1st-order diffracted reflection light and the0th-order diffracted reflection light are not, therefore, separatedimmediately unlike the first embodiment in which the approximatelyparallel light L2 is guided by the convex lens 15 to the reflection typehologram 13. When the 0th-order diffracted reflection light as well asthe 1st-order diffracted reflection light enters the liquid crystaldisplay 11, the image light has the uneven intensity, and in the case ofproviding the color image, color shading also occurs.

In the image display apparatus 3 of the embodiment, in order to avoiddeterioration of the quality of the image due to the 0th-orderdiffracted reflection light, the reflection type hologram 330 is set sothat the 1st-order diffracted reflection light and the 0th-orderdiffracted reflection light are separated as immediately as possible. Itis desirable from a viewpoint of the quality of the image that theliquid crystal display 11 is arranged farther from the reflection typehologram 330 than the position where the 1st-order diffracted reflectionlight and the 0th-order diffracted reflection light are separated. Inorder to avoid the enlargement of the apparatus, however, the liquidcrystal display 11 is arranged in a vicinity of the separating position.

In order to separate the 0th-order diffracted reflection light from the1st -order diffracted reflection light immediately, the LED 12 and thereflection type hologram 330 are set so that one angle on the side ofthe liquid crystal display 11 out of two angles formed by a principalray of the divergent light L1 from the LED 12 and the reflection typehologram 330 (they are supplementary angles) becomes an acute angle. Asa result, not less than half of the 0th-order diffracted reflectionlight advances to a direction separated from the liquid crystal display11, and even the ray which is the closest to the liquid crystal display11 is separated from the 1st-order diffracted reflection light in aposition closer to the reflection type hologram 330.

Such a structure can reduce the 0th-order diffracted reflection lightwhich enters the liquid crystal display 11 to be slight, or caneradicate it, thereby preventing the deterioration of the quality of theimage due to the mixing of the 0th-order diffracted light.

The method of manufacturing the reflection type hologram 330 is the sameas that in the second embodiment (FIG. 5). The pinhole and the lenswhich change the laser beam La on the side of the photosensitivematerial 330 b into the divergent light are arranged on the positionprovided with the LED 12 at the time of use, and an optical path withsmall diameter of the laser beam for guiding the light to the pinholeand the lens are made to coincide with the optical path of the principalray of the LED 12.

The optical structure of the image display apparatus 4 according to afourth embodiment is schematically shown in FIG. 7. The image displayapparatus 4 defined by the image display apparatus 2 according to thesecond embodiment is structured so that the 1st-order diffractedreflection light and the 0th-order diffracted reflection light(reflected light from the surface of the hologram 13) of the reflectiontype hologram 430 having a positive optical power have differentpolarizing properties, and only the 1st-order diffracted reflectionlight is guided to the liquid crystal display 11 by utilizing adifference between the polarizing properties. Concretely, theillumination light L1 from the LED 12 is first linearly polarized lightLp1, and the 1st-order diffracted reflection light of the illuminationlight L3 from the reflection type hologram 430 is second linearlypolarized light Lp2 whose polarizing direction crosses perpendicularlyto the polarizing direction of the first linearly polarized light Lp1.The surface reflected light is still the linearly polarized light Lp1.

A first polarizing element 17 is arranged between the LED 12 and thereflection type hologram 430, and a second polarizing element 18 isarranged between the reflection hologram 430 and the liquid crystaldisplay 11. A ¼ wavelength plate 19 is bonded to the surface of thereflection type hologram 430. On the first polarizing element 17 theillumination light L1 as a nonpolarized light from the LED 12 is onlythe first linearly polarized light Lp1, and a second polarizing element18 shields the first linearly polarized light Lp1 so as to transmit onlythe second linearly polarized light Lp2. Since refraction indexes of thereflection type hologram 430 and the ¼ wavelength plate 19 are barelydifferent, reflected light (0th-order diffracted reflection light) isbarely generated on the surface of the hologram 430. A polarizingdirection of the surface reflected light of the ¼ wavelength plate 19does not change, and the 1st-order diffracted reflection light transmitsthrough the ¼ wavelength plate 19 twice so that the polarizing directionis turned by 90°. As a result, only the 1st-order diffracted reflectionlight is guided to the liquid crystal display 11.

As the first polarizing element 17, an absorption polarizing filter, areflection polarizing filter, a polarizing beam splitter or the like canbe used, but particularly the reflection polarizing filter ispreferable. This because the first linearly polarized light Lp1 whichreturns to the LED 12 can be reutilized, and the use efficiency of thelight is heightened. As the second polarizing element 18, an absorptionpolarizing plate, a reflection polarizing plate and the like can beused. A polarizing plate (not shown) on the incident side provided onthe liquid crystal display 11 can be commonly used as the secondpolarizing element 18.

The reflection type holograms 13, 230, 330, 430 of the image displayapparatuses 1 through 4 according to the embodiments can have theisotropic diffusing properties in which diffusibility (diffusing angle)is uniform in all directions or can have the anisotropic diffusingproperties in which the diffusibility differs according to directions.The diffusing properties of the reflection type holograms 13, 230, 330,430 are determined by diffusing properties of the diffusing plate 16(FIGS. 2 and 5) which is used for the manufacturing, and a direction anda degree of the diffusing anisotropy of the reflection type holograms13, 230, 330, 430 can be adjusted depending on the setting of theproperties and the arrangement direction of the diffusing plate 16.

FIG. 8A shows a sectional shape of the illumination light L3 which isguided to the liquid crystal display 11 in the case where the reflectiontype holograms 13, 230, 330, 430 have the isotropic diffusingproperties. FIG. 8B shows an example of the sectional shape of theillumination light L3 in the case where the reflection type holograms13, 230, 330, 430 have the anisotropic diffusing properties. In theexample of FIG. 8B, the diffusing angle of the horizontal direction(direction x) of the viewer is set to be larger than the diffusing anglein the vertical direction (direction y).

FIG. 9 shows a size of the observation pupil P and a positionrelationship between the observation pupil P and the eye E of the viewerwhen the image display apparatus in which the diffusing properties ofthe reflection type holograms 13, 230, 330, 430 are set as shown in FIG.8B are used on both the right and left eyes. As shown in FIG. 9A, theobservation pupil P is large in the horizontal direction, and even ifthe gap between the two apparatuses is uniform, the observation of animage is easy for many viewers whose gaps of the right and left eyes Eare different. As shown in FIG. 9B, the observation pupil P is small inthe vertical direction, and light which advances to a direction where apossibility that the eye E is positioned is weak is removed, so that thelight can be utilized efficiently for the observation of the image. Theobservation pupil P is larger in the vertical direction than the casewhere the reflection type holograms 13, 230, 330, 430 do not have thediffusing properties, and even if the relative position between theapparatus and the eye E slightly fluctuates at the time of use, omissiondoes not occur on the image to be observed.

A diffracting angle (diffusing properties) of the reflection typeholograms 13, 230, 330, 430 may be enlarged in a direction of a nodalline between an plane and the reflection type holograms 13, 230, 330,430. The plane includes the center of the liquid crystal display 11, thelight emitting diode 12 (an emitting point) and the center of thereflection type holograms 13, 230, 330, 430. As a result, even if aslight error is generated in the alignment of the reflection typeholograms 13, 230, 330, 430 with respect to the liquid crystal display11 and the alignment of the light emitting diode 12 with respect to thereflection type holograms 13, 230, 330, 430, the eye of the viewer fallswithin the observer pupil, thereby lowering the accuracy necessary forassembly and thus improving the manufacturing efficiency.

When the reflection type holograms 13, 230, 330, 430 are manufactured,in stead of that the diffusing plate 16 is arranged close to the primarybodies 13 a, 230 a, 330 a, 430 a as shown in FIGS. 2 and 5, an image 16′of the diffusing plate 16 is formed by the convex lens 20 or the like asshown in FIG. 10A-D, and the laser beam after the imaging and the laserbeam before the imaging may be used as one laser beam Lb. As a result,the reflection type holograms 13, 230, 330, 430 reproduce the image 16′of the diffusing plate 16.

As sown in FIG. 10A, when the image 16′ of the diffusing plate 16 isformed between the diffusing plate 16 and the primary bodies 13 a, 230a, 330 a, 430 a, light which is equivalent to the light arrived from thediffusing plate positioned farther than the reflection type hologram 13is obtained at the time of use. The lens 21 for changing the laser beamLa into the divergent light is, as explained above, arranged in theposition provided with the LED 12 at the time of use. In FIG. 10A andFIGS. 10B through 10D described below, when the reflection type hologram13 is manufactured, a lens for generating a parallel light may be usedsuitably.

As shown in FIG. 10B, when the image 16′ of the diffusing plate 16 isformed on the primary bodies 13 a, 230 a, 330 a, 430 a, the light whichis equivalent to the light arrived from the diffusing plate positionedon the reflection type holograms 13, 230, 330, 430 is obtained. As shownin FIG. 10C, when the image 16′ of the diffusing plate 16 is tried to beformed after transmitting through the primary bodies 13 a, 230 a, 330 a,430 a, the light which is equivalent to the light arrived from thediffusing plate positioned between the reflection type holograms 13,230, 330, 430 and the liquid crystal display 11.

As shown in FIG. 10D, when the image 16′ of the diffusing plate 16 isformed in the position provided with the liquid crystal display 11 atthe time of use, the light which passes through a vicinity of the liquidcrystal display 11 is eliminated, thereby improving the light utilizingefficiency.

In the method shown in FIGS. 10A, 10C, 10D, since the image 16′ of thediffusing plate 16 is spatially separated from the primary bodies 13 a,230 a, 330 a, 430 a, the state such that the uneven intensity of thetransmitted light on the diffusing plate 16 is slackened is recorded onthe primary bodies 13 a, 230 a, 330 a, 430 a, and the reflection typeholograms 13, 230, 330, 430 for reproducing the state are obtained. Asmentioned above, therefore, the high-quality image without less unevenbrightness can be provided in comparison with the case where theillumination light is diffused by the transmission diffusing plate whichis obtained by roughing the surface of the transparent substrate.

A part of the optical system of the image display apparatus 5 accordingto a fifth embodiment will be schematically shown in FIG. 11. The imagedisplay apparatus 5 has the liquid crystal display 11, the LED 12 andthe reflection type hologram 530. The divergent light L1 from the LED 12is guided directly to the reflection type hologram 530. The reflectiontype hologram 530 has the diffusing properties similarly to theabove-mentioned embodiments, but diffracts and reflects the illuminationlight which enters the same point Q on the reflection type hologram 530,at angles different according to wavelengths. When a wavelength of alight beam A1 in the drawing, for example, is designated by λ1, and awavelength of a light beam A2 is designated by λ2, λ1≠λ2.

The method of manufacturing the reflection type hologram 530 isschematically shown in FIG. 12. The laser beam La emitted from thephotosensitive material 530 b is supposed to be the divergent light.Differently from the above-mentioned embodiments, the diffusing plate 16is arranged on the optical path of the laser beam La as the divergentlight. As a result, the reflection type hologram 530 which makes thediffracting reflection angle different according to the wavelengths canbe obtained.

The entire optical structure of the image display apparatus 5 isschematically shown in FIG. 13. In the image display apparatus 5, thereflection type hologram 14 a is used as the eyepiece optical system 14.In order to avoid confusion between the reflection type hologram 14 aand the reflection type hologram 530 for guiding the illumination light,the reflection type hologram 14 a is called as an eyepiece hologram. Theeyepiece hologram 14 a is provided in a flat plate type prism 22, andreflect the image light which enters the prism 22 from its end surfaceto guide the light to the eye E of the viewer, so as to provide theenlarged virtual image of the image.

The method of manufacturing the eyepiece hologram 14 a is schematicallyshown in FIG. 14. The laser beam Ld as the parallel light and the laserbeam Lc as the divergent light are emitted to the photosensitivematerial 14 b, so that the interference bands of them are recorded. Thelaser beam Lc is emitted from a position corresponding to the eye of theviewer (observation pupil), and the laser beam Ld is emitted from adirection which is the same as or opposite to the direction of the imagelight. This provides the eyepiece hologram 14 a having a function forproviding the enlarged virtual image.

In the eyepiece hologram 14 a manufactured in such a manner, however, adifference is made on a peak wavelength of the diffracting reflectionefficiency depending on the incident angles. The three light beams B1,B2, B3 shown in FIG. 13, for example, enter the eyepiece hologram 14 aat different incident angles, and the peak wavelengths of theirdiffracting reflection efficiency are also different. When the size ofthe observation pupil P is supposed to be 3 mm, a distance from theobservation pupil P to the center of the eyepiece hologram 14 a to be 20mm, a thickness of the eyepiece hologram 14 a to be 20 μm, refractiveindex percentage modulation to be 0.01, a center wavelength of theillumination to be 532 nm, and an angle of view of the image to beprovided (up-down direction in FIG. 13) to be 10°, the difference in thepeak wavelength of the diffracting reflection efficiency between thelight beams B1 and B3 reaches 40 nm. This is similarly applied to lightbeams C1 and C3.

On the other hand, the light beams B1 and C1 in which the incidentangles with respect to the eyepiece hologram 14 a are equal have nodifference in the peak wavelength of the diffracting reflectionefficiency. This is similarly applied to the light beams B2 and C2 andthe light beams B3 and C3.

In order to enlarge the observation pupil P while using the eyepiecehologram 14 a having such properties, it is necessary that theillumination light has a certain wavelength width, and the light beamswhich enter the eyepiece hologram 14 a at different angles havewavelength distribution according to the diffracting reflectionefficiency. The reflection type hologram 530 which guides theillumination light to the liquid crystal display 11 is, therefore,provided with the properties which make a difference in the wavelengthof the light advancing to different directions. The reflection typehologram 530 can increase a light quantity in a wavelength at which thediffracting reflection efficiency on the eyepiece hologram 14 a becomeslow, thereby offsetting the above properties of the eyepiece hologram 14a.

The light from the LED 12 which enters one point Q of the reflectiontype hologram 530 is diffracted and reflected to different directionsaccording to wavelengths, and for example, it is divided into the lightbeam B1 and the light beam C2. The light beams B1 and C2 pass throughthe liquid crystal display 11 and the eyepiece hologram 14 a so as toform some parts of the observation pupil P. The light which entersanother point of the reflection type hologram 530 and divided similarlyforms some part of the observation pupil P. Quantities of these lightare sufficient, and thus the large observation pupil P can be obtainedentirely.

An outline of the glass-type image display apparatus 5 is shown in FIG.15. A frame 23 which houses the liquid crystal display 11, the lightemitting diode 12 and the reflection type hologram 530 is attached to anupper edge of the prism 22 corresponding to the lens of the glasses. Theeyepiece hologram 14 a is provided on a center portion of the prism 22.The frame 23 is connected with a cable 24 for giving an image signal, acontrol signal and the like from a controller, not shown.

The cross section of the image display apparatus 5 is shown in FIG. 16.The upper end of the prism 22 has a wedge-shaped section, and the lightemitting diode 12, the reflection type hologram 530 and the liquidcrystal display 11 are arranged in the frame 23 so that the image lightfrom the liquid crystal display 11 enters the end surface of the prism22 slantingly. The light from the liquid crystal display 11, which goesinto the prism 22 from its end surface, is totally reflected from thetwo surfaces of the prism 22 and simultaneously advances downward, so asto be guided to the eye by the eyepiece hologram 14 a.

The prism 22 is transparent, and the viewer mounted with the imagedisplay apparatus 5 can observe an outer world via the prism 22. Alsothe eyepiece hologram 14 a transmits light from the outer world, andprovides the enlarged virtual image of the image which is overlappedwith the image of the outer world.

That the diffusing properties are provided to the reflection typehologram for guiding the illumination light to the display element canbe applied also to a projector for enlarging an image and projecting theenlarged image on a screen. In general, in order to widen the anglerange in which the image can be observed, the screen is provided withthe diffusing properties, but the reflection type hologram for guidingthe illumination light is also provided with the diffusing properties,thereby further widening the angle range in which the image on thescreen can be observed. In order to enable an image of the outer worldbehind the screen also to be observed, even when the screen is atransparent plate without the diffusing property, the angle range inwhich the image can be observed can be widened by changing theillumination light into the diffused light. Instead of the eyepieceoptical system 14, only a projecting optical system for imaging thelight showing the image from the liquid crystal display 11 on the screenis provided, so that such a projector can be obtained.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention, theyshould be construed as being included therein.

1-17. (canceled)
 18. An image display apparatus comprising: a lightsource section for supplying the illumination light; a display elementfor modulating a given illumination light into an image light showing animage; a first reflection type hologram for diffracting and reflectingthe illumination light from the light source section so as to guide theillumination light to the display element, the first reflection typehologram having diffusing properties; a second reflection type hologramfor guiding an image light from the display element to an eye of aviewer so as to provide an enlarged virtual image of the image; and atransparent plate-shaped prism for holding the second reflection typehologram, wherein the image display apparatus is used with the secondreflection type hologram placed in front of the eye of viewer and withthe transparent plate-shaped prism facing the eyes, the image light fromthe display element enters the transparent plate-shaped prism at an endsurface thereof so as to be guided to the second reflection typehologram and is guided to the eye of the viewer by the second reflectiontype hologram as an enlarged virtual image of the image, and a lightfrom an outer world transmits through the plate-shaped prism and thesecond reflection type hologram so as to be guided to the eye, therebythe viewer can simultaneously see an image of the outer world and thevirtual image.
 19. The image display apparatus as claimed in claim 18,the light source section supplying the illumination light with aplurality of wavelengths, wherein a light component of the illuminationlight which enters the reflection type hologram at a same point arediffracted and reflected by the reflection type hologram at differentangles according to a wavelength of the light component.
 20. The imagedisplay apparatus as claimed in claim 19, wherein a peak wavelength ofdiffraction efficiency of the first reflection type hologram differsfrom that of the second reflection type hologram.
 21. The image displayapparatus as claimed in claim 18, wherein the image display apparatus isa glass-type image display apparatus.